Table of contents

The study of the electric double layer (solid/liquid interface) is of crucial importance for an in-depth understanding electrochemical processes. However, the electrochemical interface (or interphase) is usually buried under a barrier of bulk water which is blocking direct access for most analytical tools. One way to enable direct access is e.g. the emersed electrode concept. For this, electrodes are emersed under potential control from the electrolyte into an inert atmosphere. It was shown in prior works for emersion into UHV [1] and into humid nitrogen [2] that the electrochemical double layer seems to be kept more or less intact during this process. However, thus an electrochemical double layer is obtained that is vertically confined to just one or two monolayers, depending on the humidity of the environment. The Kelvin probe technique can be employed for measuring the resulting electrode potential of such emersed electrodes [3], often providing a linear correlation between applied potentials during the electrochemical polarization and the potentials determined after emersing the sample from the electrolyte [2]. An open question is the role of the amount of water molecules in the double layer on the electrode potential [1,2]. A powerful technique for the investigation of the structure of the water molecules in the double layer region is infrared spectroscopy. Since infrared spectroscopy techniques (using an ATR set-up) can also be applied for characterization of the electrode immersed bulk electrolyte [4], a direct comparison between these two cases is possible.

For this work, NaClO₄ and H₂SO₄ diluted aqueous solutions were used as electrolytes, which both present anions with similar specific adsorption [5]. The Kelvin probe tip was calibrated against an Ag/AgCl/KCl homemade reference electrode [6]. Different atmosphere conditions (relative humidity) were used and their effect on the emersed electrode was investigated by Kelvin probe and IR spectroscopy. Furthermore, studies employing Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS) technique have been conducted in order to detect the presence of anions and cations in the electrochemical double-layer and also for in operando studies with different atmosphere conditions.

Literature:

[1] H. Neff, R. Kötz, J. Electroanal. Chem. 1983, 151, 305.

[2] Z. Samec, B. W. Johnson, K. Doblhofer, Surf. Sci. 1992, 264, 440.

[3] M. Rohwerder, F. Turcu, Electrochim. Acta 2007, 53, 290.

[4] K. Ataka, T. Yotsuyanagi, M. Osawa, J. Phys. Chem. 1996, 100, 10664.

[5] D. D. Bodé Jr., T. N. Andersen, H. Eyring, J. Phys. Chem. 1967, 71, 792.

[6] M. Uebel, A. Vimalanandan, A. Laaboudi, S. Evers, M. Stratmann, D. Diesing, M. Rohwerder, Langmuir 2017, 33, 10807.

Conductive electrodes functionalized with molecularly well-defined species are emerging as effective active materials for a wide range of applications, from electrocatalytic CO₂ and O₂ reduction, to high performance lithium-sulfur batteries.^1–3 For redox-active species that participate in proton-coupled electron transfer (PCET), the extent of conjugation, applied potential, and distance between the redox center and electrode determine the potential and electric field experienced at the site of electron transfer. This electric field in turn determines the pH-dependent energetics of electron transfer (i.e. Pourbaix behavior) and the rate PCET. We discuss how measurements of PCET to redox-active species functionalized to carbon electrodes can yield insight into the potential and electric field profiles within the electrode-electrolyte double layer. Systematically increasing the distance between the redox-active moiety and the electrode results in the moiety experiencing a progressively weaker electric field, as determined by changes to PCET behavior. Based on these results, we propose strategies to quantify the strength of the electric field as a function of this distance. This work can inform strategies for effective pH modulation at electrified interfaces in ways that can enhance electrocatalytic processes and other applications, such as electrochemical CO₂ capture based on pH swing.

References

1. Ren, G. et al. Porous Core-Shell Fe3C Embedded N-doped Carbon Nanofibers as an Effective Electrocatalysts for Oxygen Reduction Reaction. ACS Applied Materials and Interfaces8, 4118–4125 (2016).

2. Zhang, S., Fan, Q., Xia, R. & Meyer, T. J. CO2 Reduction: From Homogeneous to Heterogeneous Electrocatalysis. Accounts of Chemical Research53, 255–264 (2020).

3. Zhao, C.-X. et al. Semi-Immobilized Molecular Electrocatalysts for High-Performance Lithium–Sulfur Batteries. Journal of the American Chemical Society143, 19865–19872 (2021).

Several families of electrochemical energy conversion and storage devices (e.g., fuel cells and metal-air batteries) exploit the oxygen reduction reaction (ORR) in their operation. Very often, the ORR is the slowest electrochemical process in these systems and plays the most relevant role to degrade their energy conversion efficiency. Thus, to devise high-performing ORR electrocatalysts (ECs) is of crucial importance for practical applications. Unfortunately, to understand in detail the operating mechanism of an ORR EC is typically complex and time-consuming, requiring the implementation of a broad spectrum of advanced techniques and extensive data analysis. Consequently, there is a strong need to design simple methods capable to yield information on the most critical features of an ORR EC for screening purposes. In this regard, cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) is a very popular approach. The latter allows for the facile study of the kinetics of an electrochemical process as promoted by an EC and minimizes the influence of complex spurious phenomena (e.g., charge and mass transport).

In this contribution a general method is discussed allowing for the correlation of the outcome of conventional electrochemical experiments with the critical features determining the performance of an ORR EC. Such features include prominently: (i) the kinetic activation barrier of the ORR; and (ii) the accessibility of O₂ to the active sites. The proposed method: (i) adopts the CV-TF-RRDE setup; (ii) does not lean on the simplifications associated with the conventional Butler-Volmer kinetic description of electrochemical processes; and (iii) does not make assumptions on the specific features of the EC. As a result, the proposed method allows to compare accurately the kinetic performance of ORR ECs exhibiting a completely different chemistry. Finally, it is shown that the figure of merit considered in this method, E(j_Pt(5%)), is much more accurate than other popular figures of merit to gauge the ORR such as the half-wave potential E_1/2.

Acknowledgements

This research has received funding from (a) the European Union's Horizon 2020 research and innovation program under grant agreement 881603 (b) the project 'Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells' funded by EIT Raw Materials, (c) Alkaline membranes and (platinum group metals)-free catalysts enabling innovative, open electrochemical devices for energy storage and conversion d AMPERE, FISR 2019 project funded by the Italian Ministry of University and Research, and (d) the project 'Hierarchical electrocatalysts with a low platinum loading for low-temperature fuel cells d HELPER' funded by the University of Padova. PJK and IAR (University of Warsaw) were supported in part by the National Science Center (NCN, Poland) under Opus Project 2018/29/B/ ST5/02627.

Redox reactions on electrode surfaces have been widely applied to fundamental technologies such as energy conversion and storage, metal refining, and electronics. Recently, it has become clear that local structures at the single-atom and single-molecule level affect the efficiency and selectivity of redox reactions [1]. EC-STM, a combination of scanning tunneling microscopy (STM) and electrochemical (EC) measurements, allows direct observation of electrochemical interfaces with high spatial resolution [2], and we aimed to observe the redox reactions of single molecules using this technique. As a target molecule, ferrocene (Fc) was coupled to a tripodal base molecule [3], which was known to form an ordered self-assembled monolayer (SAM) on a gold electrode. By preparing mixed SAM with the molecules with and without Fc moieties, we could stably deposit Fc species on the Au (111) electrode in an isolated and dispersed state, which enables us to observe clear changes in the EC-STM images upon the redox reaction of the single Fc derivative.

A mixture of 8,13-trimercaptotriptycene (Trip) and its Fc derivative (Fc-Trip) was deposited on an Au(111) electrode (the molecules were provided by the Fukushima Lab., Chemical Biology Laboratory, Tokyo Institute of Technology, and the Suzuki Lab., Graduate School of Science, Hokkaido University). Cyclic voltammetry (CV) was performed with a potentiostat (HZ-7000, Hokuto Denko) at a sweep rate of 100 mV/s in 0.1 M HClO₄ solution. STM measurements were performed using an MS-10 STM (Bruker), controlled by a NanoScope V (Bruker). A Pt/Ir wire coated with Apiezon wax was used as a probe in the home-made electrochemical cell. The sample potential (E_sample) was varied from 0.2 V (vs Ag/AgCl) to 0.4 V while the potential difference between the tip and the sample was kept at 0.3 V.

The CV results are shown in Fig. 1 (a), where the peak originating from the redox reactions of Fc-Trip is observed (dotted line is only for Trip), indicating that the redox potential of Fc-Trip is ~0.3 V. Since the full width at half maximum of the peak is about 0.1 V, Fc-Trip do not interact with each other and are considered to be isolated and dispersed on the Au(111) electrode [4]. Corresponding EC-STM images are shown in Fig. 1 (c)-(e). While the bright spots derived from Fc-Trip are observed at E_sample = 0.2 V (the tip potential E_tip is 0.5 V), as indicated by white circles, the bright spots almost disappeared at E_sample = 0.4 V (E_tip = 0.7 V). We confirmed that the change of the height is reversible with respect to the applied potentials, as shown in Fig. 1(c) and 1(e). The height of the bright spot in the EC-STM image reflects the electron transfer rate through the redox molecule between the tip and the sample.

When the sample potential is set to 0.2 V, Fc-Trip is formally in the reduced state, but the electrons are readily transferred to the tip regulated at 0.5 V. On the other hand, when the sample potential is set to settle the oxidized state (0.4 V), the molecule cannot pass electrons to the high potential tip. Therefore, in the case of Fig. 1(d), the current between the tip and the sample is not increased with the presence of the Fc moiety, leading to the apparent disappearance of spots. We noticed that some bright spots did not disappear at E_sample = 0.4 V (an arrow in Fig. 1(d)), which suggests that there is heterogeneity in the redox reactivity. In the future, we plan to investigate the heterogeneity by combining spectroscopic techniques with the EC-STM measurement.

[1] J. H. K. Pfisterer et al., Nature,74, 549 (2017)

[2] Y. Yokota et al., J. Phys. Chem. C, 111, 7561 (2007)

[3] F. Ishiwari et al., J. Am. Chem. Soc., 141, 5995 (2019)

[4] C. E. D. Chidsey et al., J. Am. Chem. Soc., 112, 4301 (1990)

Figure 1

The importance of hydrogen evolution reaction (HER) in engineering and energy conversion related applications grows stronger and it is important to develop better catalysts to enhance the efficiency of the catalytic process. The introduction of computational chemistry (e.g., DFT) into the field of electrochemistry allows general prediction and understanding of activity trends. Great progress is made in modelling the electrochemical double layer under potentiostatic control [1]. However, a reliable and feasible experimental methodology providing in-depth experimental data for improving the theoretical model is lacking, as the electrochemical interface is usually buried under bulk electrolyte, which prevents most important surface analytical techniques to be applied for its investigation. However, it could be recently shown that a hydrogen electrode forms on a palladium surface even in nitrogen atmosphere, even at very low humidity [2] . The electrochemical double layer of this "hydrogen electrode in the dry" is confined just 1-2 monolayers of adsorbed water. However, it is still fully polarizable by adjusting the hydrogen activity from the backside of the electrode and it is even possible to measure full current-potential dependencies for reactions such as e.g. oxygen reduction [3]. Hence, this could be revolutionary new approach for investigating electrochemical electrodes in operando by standard surface analytical techniques. For this however, it needs to be investigated in how far this approach can be also applied beyond palladium. Since the potential of such "electrodes in the dry" cannot be measured with standard reference electrode a Kelvin probe is used instead [2,3]. Basically, the Kelvin probe measures the Volta potential difference between sample and probe (ψ^KP - ψ^S), and upon suitable calibration such measurement provides the electrode potential of the sample.

Evers et.al has proposed the "Hydrogen electrode in dry" concept based on adjusting the hydrogen activity (cathodic charging) from one side of a palladium sample and measuring the work function on the other side using Kelvin probe [2]. In this work, the hydrogen electrode formation in dry (<0.1% R.H) as well as humid condition (>95% R.H) for Palladium (Pd), iridium (Ir) and gold (Au) was carried out using scanning Kelvin probe method by electrochemical charging of hydrogen from entry side and subsequently measuring the Volta potential on exit side (see fig.1). The measured potential on the exit side can be related to electrode potential. Palladium showed a 1:1 correlation between applied potential and measured potential at higher hydrogen activities (between 0 to 300 mV_SHE) by establishing electrochemical equilibrium in both conditions, in accordance with [1]. Though the partial pressure of water is very low (0.23mbar) in dry condition, still the hydrogen electrode formation occurred on the palladium. In the case of Ir, it shows 1:1 correlation only in humid condition and not in dry condition. Further, the hydrogen electrode formation on Ir at different partial pressures of water was investigated by controlling the humidity on the exit side of the sample and the obtained results can be attributed to slight differences of thickness of the water layer in the monolayer and even sub-monolayer range. A linear decrease in the potential was observed when increasing the humidity from low to high humidity and the correlation between potential applied from the back and potential measure in the "dry" shows a decrease in responsiveness with decreasing humidity. No correlation was observed for iridium in dry conditions and it can be attributed to presence of insufficient amount of adsorbed water. For gold no correlation was observed in both the conditions. However, a slight behavior was observed at very high activities of hydrogen. This may be due to the initial hydrophobicity of gold, which has first to overcome by a formation of a double layer. On the whole, the effect of water layer adsorption on different metals and its influence on the establishment of a hydrogen electrode at a dry surface was studied.

References:

[1] F. Deißenbeck, C. Freysoldt, M. Todorova, J. Neugebauer, S. Wippermann, Dielectric Properties of Nanoconfined Water: A Canonical Thermopotentiostat Approach, Phys. Rev. Lett. 126 (2021) 136803.

[2] S. Evers, M. Rohwerder, The hydrogen electrode in the dry: A Kelvin probe approach to measuring hydrogen in metals, Electrochem. Commun. 24 (2012) 85–88.

[3] X. Zhong, M. Schulz, C.H. Wu, M. Rabe, A. Erbe, M. Rohwerder, Limiting Current Density of Oxygen Reduction under Ultrathin Electrolyte Layers: From the Micrometer Range to Monolayers, ChemElectroChem. 8 (2021) 712–718.

Figure 1

Single entity electrochemistry encompasses a broad collection of techniques that are used to detect and characterize single, freely diffusing analytes in solution^1,2. A particular scheme in this field, "blocking" nanoimpacts, has expanded the scope of electrochemistry to the study of redox-inert materials. Here, as a particle adsorbs, it prevents charge exchange between the microelectrode with a freely diffusing electroactive redox mediator. The interpretation of these blocking results, however, is often complicated by "edge effects," in which enhanced mass transport to the edges of microelectrodes leads to uneven current distributions^3,4. To overcome this problem we introduce here the use of electrochemical amplification⁵. That is, we employ electrocatalytic amplification to drive a current increase, moving the rate limiting step in current generation away from the electrode surface, reducing the geometric impact of the electrode's edges. Finite element simulations indicate that the rapid chemical kinetics introduced by this approach contributes to the amplification of the electronic signal to restore analytical precision and reliability detect and characterize the heterogeneity of nanoscale electro-inactive materials. Using this approach, which we have termed "electrocatalytic interruption," we achieve significantly improved precision in the determination of particle size distributions.⁶

References

1. Quinn, B. M.; van't Hof, P. G.; Lemay, S. G. Time-Resolved Elec-trochemical Detection of Discrete Adsorption Events. J. Am. Chem. Soc. 2004, 126 (27), 8360–8361. https://doi.org/10.1021/ja0478577.

2. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001.

3. Compton, R. G.; Banks, C. E. Understanding Voltammetry (Third Edition); World Scientific, 2018.

4. Deng, Z.; Elattar, R.; Maroun, F.; Renault, C. In Situ Measurement of the Size Distribution and Concentration of Insulating Particles by Electrochemical Collision on Hemispherical Ultramicroelectrodes. Anal. Chem. 2018, 90 (21), 12923–12929. https://doi.org/10.1021/acs.analchem.8b03550.

5. Bonezzi, J.; Boika, A. Deciphering the Magnitude of Current Steps in Electrochemical Blocking Collision Experiments and Its Implica-tions. Electrochimica Acta 2017, 236, 252–259. https://doi.org/10.1016/j.electacta.2017.03.090.

6. Chung, J.; Hertler, P.; Plaxco, K.W.; Sepunaru, L. J. Am. Chem. Soc. 2021, https://doi.org/10.1021/jacs.1c04971.

Figure 1

Solar energy can be absorbed via surface plasmon resonance (SPR) to promote excitation of energetic, or "hot", charge carriers that can be locally transferred or thermally dissipated to augment photocatalytic processes. The "hot" carriers can selectively drive energy-intensive photoelectrochemical reactions at low temperatures by activating adsorbed reactants and accelerating surface kinetics. Plasmonically-sensitized nanocatalysts were investigated for their photocatalytic and photoelectrochemical oxidation of ethanol, with an emphasis on carbon-carbon bond cleavage, under solar simulated-light irradiation. Material approaches included the (i) SPR-functionalization of a traditional metal oxide semiconductor (TiO₂) and (ii) bimetallic nanocatalysts composed of epitaxially photodeposited catalytic Pd at targeted locations on plasmonic Au nanorods. Results are correlated with nanocatalyst morphology, composition, and homogeneity to maintain SPR-induced charge separation and mitigate carbon monoxide poisoning. Ensemble photoelectrochemical measurements were complimented with single-particle dark-field scattering and photoluminescence spectroscopies to understand the extraction and utilization of SPR-excited "hot" carriers. Ethanol oxidation was achieved, yielding a solar-driven method for low temperature, complete photo-oxidation of complex hydrocarbons via plasmonic photocatalysis.

We will discuss the use of a tobacco mosaic virus coat protein (TMVCP) as a versatile platform to synthesize a series of metallic nanomaterials and their applications in energy related electrocatalytic reactions. Taking advantage of the self-assembly properties of TMVCP under different solution conditions, nanoparticles can be embedded onto the disk protein surface or capped by protein subunits. We show that in addition to the eco-friendly synthesis merit, the as prepared materials are superior catalysts in electrocatalytic reactions. While the nanosized silver rings exhibit significant enhancement towards catalysing electrochemical carbon dioxide reduction reaction, the TMVCP-templated two platinum catalyst are promising candidates for methanol oxidation reaction. Besides electrocatalysis, potential applications of protein-metal hybrid systems in heterogeneous organic synthesis are also described. Gold and palladium nanoparticles capped by the bulky protein subunits are employed for two organic reactions and also showed remarkable kinetics.

In the case of heterogeneous catalysis (where the reactions occur onto the catalyst surface), the most promising approach in material design follows the Sabatier principle: the ability of the surface to bind adsorbates and the strength of the bonds define the reaction thermodynamics and kinetics. In this respect, the catalyst surface chemistry and structure (crystallographic orientation of the facets and/or strain) are used as levers to optimize its performance. However, it is largely accepted that the structure of PEMWE anode and PEMFC cathode catalysts does not remain unaltered over the long term operation due to harsh (electro)chemical conditions and it is of a high interest to understand the structural changes during the electrocatalysis. In this contribution we use X-ray diffraction techniques to qualitatively and quantitatively describe those changes and shed light on the strain evolution in nanostructured catalysts during operation in the electrochemical environment.

Using the newly upgraded 4th generation EBS source at European Synchrotron Radiation Facility, we study Pt and Pd catalysts in liquid half cells at various potentials. In the Pd case we focus on the phase and structural dynamics during H insertion, allowing to unveil the electrochemically-driven Pd hydrides phase transition, which was until now mostly investigated in the gas phase. The fine XRD patterns measured operando provided the first detection of theoretically predicted supersaturated and undersaturated metastable states, involved in a core-shell mechanism of the phase transition (Figure 1) [1].

In the case of Pt nanocatalyst, monitoring the structural features of the catalyst during cyclic voltammetry permits studying adsorption and oxidation processes. Deconvoluting both effects (adsorption and oxidation) is possible by following different parameters of the analyzed XRD patterns with high temporal resolution. Such experiments reveal important dynamic trends linked to the surface oxidation and degradation, directly impacting the output of accelerated stress tests (AST) [2]. Carefully determining the charge passed through the circuit during the CV also allows to establish a near-linear correlation between the expansion of the bulk lattice parameter of Pt nanoparticles and their oxide surface coverage. Since the latter is known to be a descriptor of catalysts activity toward numerous reactions (oxygen reduction and fuel oxidation), the measurement of the lattice parameter according to this simple diffraction approach allows experimental access to this descriptor during operando measurements in device-relevant sample environments [2]. This constitutes a main advantage compared to spectroscopic techniques and is foreseen to find a wide range of applications, as demonstrated by measuring the structural behavior of the state-of-the-art PtNiIr octahedra catalyst in operating PEMFC. In this case, the results allow correlating the degradation of the catalysts cycled to different lower potentials with the oxidation and strain dynamics. Based on this data, the operating conditions for this catalyst can be determined, further defining the application of this material.

[1] Chattot, R. et al.,J. Am. Chem. Soc.143(41), 17068–17078 (2021).

[2] Martens, I. et al.,ACS Appl. Energy Mater.2 (11), 7772-7780 (2019).

Figure 1

During the recent SARS-CoV-2 pandemic, lateral flow assays (LFAs) have written an indescribable story of success, as they provide means for decentralized, low-cost, and easy-to-use testing. However, LFAs in their most common form support only qualitative results and their sensitivity and limit of detection is significantly limited by the colorimetric readout method. In contrast, single-impact electrochemistry offers the possibility to quantify species beyond picomolar concentrations by recording individual species collisions with a biased microelectrode. Within this work, we investigate the integration of stochastic sensing into a LFA-architecture by combining a wax-patterned microchannel with a microelectrode array in order to detect silver nanoparticles by their oxidative dissolution. Here, we demonstrate the possibility to resolve individual nanoparticle collisions in a paper-based microchannel using a simplified reference-on-chip setup. Furthermore, we simulated a lateral-flow sensor, by flushing previously dried silver nanoparticles towards the electrode array, where the particles are subsequently detected. This proof-of-principle illustrates that single-impact electrochemistry might be a promising technique to extend the capability of LFAs. Especially, the integration of functionalized nanoparticle labels could enable the rapid and on-site detection of very dilute species with exceptional sensitivity.

Physical Reservoir Computing (PRC) has recently attracted significant attention as a computational method suitable for the edge AI computing, which requires both the high performance of information processing and energy conservative operation. There are two standard methods for evaluating PRC performance: the short-term memory (STM) task for the memory capacity and the parity check (PC) task for the nonlinear conversion capacity. We have developed PR device utilizing Faradaic currents generated by the redox reaction of metal ions in ionic liquids (ILs) and the impact of metal ions in ILs on the STM characteristics has been evaluated by comparing the RC device using metal-ion doped IL with that using non-doped (pure) IL ^[1]. In this study, we investigated the effect of Faradaic current on the STM and PC tasks by extracting the Faradaic current from the output signal when the triangular shaped input voltage pulse was applied to the PR device. It was found out that the peak shaped Faradaic current in the output signal improves not only the memory capacity but also the nonlinear conversion capability.

A reservoir device with a transverse Pt/SiO₂/Pt structure was prepared (Fig. 1) and solvated IL, Cu(Tf₂N)₂-Glyme(G3)=1:1 ^[2], was provided between the Pt electrodes as a reaction field where electrochemical reaction of Cu actively takes place. In order to prevent unnecessary copper deposition on the top surface of the Pt electrode, all areas other than the Pt electrode tip was covered with SiO₂. As a result, a structure that allows deposition only between the terminals was realized. Furthermore, to prevent the migration of the IL by the application of an electric field, an IL pool surrounded by a resist wall was formed by patterning a spin-coated photoresist AZ5214E with a thickness of 2 µm. Au/Ti (100/10 nm) was deposited as the contact pad. The device characteristics were evaluated by cyclic voltammetry. In addition, a response of the device to the triangular pulses was investigated by using the B1530A WGFMU (waveform generator / fast measurement unit). STM and PC tasks were used to evaluate the time series data processing ability.

In the present study, an artificial time-series data consisting of randomly connected binary data (0 and 1) was input to one of the Pt electrodes as the triangular shaped voltage pulse stream, while the other electrode was grounded. The positive and negative voltages were defined as 2bit data, '1' and '0', respectively. As shown in Fig. 2, triangular voltage pulse streams shown by the blue line were input to the reservoir device, and output current shown by red line was observed. Current peak appears when the polarity of the input signal is switched from positive to negative and vice versa (black arrow), but the peak intensity decreases when application of voltage pulses with the same polarity are continuously repeated. In addition, Cu deposition on the Pt electrode was observed. These results indicate that the origin of the peak is the Faradaic current generated by the electroactive species near the electrode. We divided output current data into two parts; the first half (yellow region), the latter half (green region). The first half corresponds to the rising part of the triangular pulse and involves the Faradaic current. On the other hand, the latter half corresponds to the descending part of the triangular pulse and is more featureless compared with the first half. We found that the accuracy of STM task was much higher when the first half was used. This tendency was also confirmed for the PC task. In PRC based on the concept of virtual nodes using a single physical device ^[3], the output signal complexity is considered to be related to the multidimensional transformation capability, which is one of indispensable property for PRC ^[4]. The present results suggest that the redox reaction of electroactive species in the IL increases the complexity of the output signal and improves the ability to extract time-series features.

We thank Mr. Hiroshi Sato for supporting device fabrication processes. A part of this work was supported by "Nanotechnology Platform Program", Grant Number JPMXPF21NM0006.

[1] D. Sato et al., MEMRISYS 2021, 4A-7 (2021).

[2] H. Yamaoka et al., Chem. Lett., 46, 1832-1835 (2017).

[3] L. Appeltant et al., Nat. Commun., 2, 468 (2011).

[4] L. Appeltant et al., Sci. Rep., 4, 3629 (2014).

Figure 1

On-chip solid-state micro-supercapacitors (MSCs) have gained widespread attention owing to their versatile benefits, including high power density and long cycle life^1,2. Recently asymmetric hybrid microsupercapacitors (AsHMSCs) introducing a charge storage mechanism based on both faradaic reaction and electric double-layer phenomena have been considered to increase the energy density. Within this context, the selection of electrode materials and their interaction at the interface with the electrolyte is a key issue to optimize the AsHMSC electrochemical performance.

In this work, nanometric (5-20 nm thick) amorphous TiO₂ films have been elaborated and characterized in liquid and solid-state electrolyte (LiPON) half-cell configurations. Interestingly, TiO₂ films after LiPON deposition exhibited a constant initial amount of intercalated lithium ions for all considered thicknesses and did not require a first activation process, in comparison to the liquid electrolyte configuration. For all considered configurations, the specific capacity extracted from cyclic voltammetry and galvanostatic cycling within [0.5-3V] potential range correspond to the theoretical value expected for the Li_x=1TiO₂ phase at low current density. Furthermore, the cooperative effects of high Li ion intercalation kinetics and Low interfacial charge transfer resistance for 5nm TiO₂ electrode (extracted from electrochemical impedance spectroscopy analysis) exhibit an outstanding specific capacity of 2mC.cm^-2 at 1µA.cm^-2, and high rate performance with 60% capacity holding ratio at 100µA.cm^-2, thus highlighting the extrinsic pseudo-capacitive behavior of the sub 10nm electrodes.

A TiO₂ 5nm/LiPON 100nm/Pt AsHMSC is successfully fabricated, which achieves an operating voltage window of 3V and a specific capacity of 0.3mC.cm^-2 at 1mA.cm^-2.In addition, the device also exhibited a 100% coulombic efficiency even after cycling for 6000 continuous charge-discharge cycles. This work offers an approach to tune the Li ion storage properties of TiO₂ by nano-engineering and gives insights into the enhancement of pseudocapacitance-assisted lithium-storage capacity.

References

1. Kyeremateng et al., Nature Nanotech 12, 7–15 (2017). https://doi.org/10.1038/nnano.2016.196

2. Sallaz et al., J. Power Sources 451, 227786 (2020), doi: 10.1016/j.jpowsour.2020.227786.

Figure 1

The engineering of carbon nanofibers (CNFs) has recently received extensive attention in the field of biosensors due to their high surface area, rich nanoscale geometries, ability to selectively detect the analyte of interest (e.g., dopamine) in the presence of interfering molecules that exist in biological environment and inherent resistance to biofouling^1-3. CNFs with the individual fiber length/size adjusted to the diffusion layer thickness can confine the analyte molecules within the nanostructures and result in the formation of a thin liquid layer, increasing their sensitivity ⁴. This points towards the potential of altering the aspect ratios and growth densities of CNFs to tune their biosensing properties. Thus, here we aim to evaluate the effect of the length, distribution, and microstructure of fibers on their electrochemistry and how this affects the sensitivity and selectivity for dopamine (DA) detection.

Herein, CNFs of varying lengths have been grown on a silicon substrate using the thin layers of Ni and Cr as catalysts. Samples with different length and distribution of CNFs have been synthesized by controlling the duration of the growth phase. The length and the detailed structure of the CNFs have been investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The lengths of CNFs were found to be in the range of 900 nm-1µM, 500-600 nM, 200-300 nM, and <150 nM for 30 mins, 10 mins, 5 mins and 1 min growth times, respectively. Cyclic voltammetry measurements showed that the double-layer capacitance (C_dl) and oxidation current (I_pa) of dopamine increase continuously with the increase in the length of fibers indicating the increase in the surface area (Figure 1A). DA reaction kinetics tend to be reversible for the longest fibers and becomes slightly more sluggish with the decrease in the fiber length. However, it should be noted that carbon nanofiber length (hundreds of nM to 1µm) may match the diffusion layer thickness at certain scan rates, causing the thin liquid layer electrochemical behavior to contribute to the total kinetics. This implies that the variations in the peak separation (ΔE_p) can be due to the combined changes in reaction kinetics and geometry. CNFs grown for 30 mins, 10 mins, 5 mins, and 1 min exhibited log I_pa vs v slope of 0.72, 0.63, 0.62 and 0.51, indicating that contribution of adsorption/thin layer formation is coming into effect with the increase in fiber length (Table 1). Washout experiments will be performed to distinguish further between thin layer and adsorption behavior affecting the electrochemistry of DA. It appears that DA reaction kinetics is fully under semi-infinite linear diffusion-control only at 1 min grown CNFs due to the relatively smooth surface of the electrode material. The selectivity of dopamine in the presence of physiological concentrations of ascorbic acid (AA) and uric acid (UA) greatly improves with the increase in the length of the fibers (Figure 1B). Electrodes with 1 min grown CNFs possessing metal particles and exhibiting little CNF growth did not show selectivity towards DA. Interestingly, with the increase in the growth time and consequently the length of the fiber, well-defined oxidation peaks of AA and DA and UA are observed. Moreover, the position of the oxidation potential appears to be affected both by the fiber length and presence/absence of other molecules, and often in the opposite directions (Table 1, Figure 1). The results discussed above point towards the significance of matching the CNF length with the scan rate used for in vivo detection of DA for optimized sensitivity and selectivity.

References

1 S. Sainio, T. Palomäki, N. Tujunen, V. Protopopova, J. Koehne, K. Kordas, J. Koskinen, M. Meyyappan and T. Laurila, Mol. Neurobiol., 2015, 52, 859–866.

2 S. Sainio, E. Leppänen, E. Mynttinen, T. Palomäki, N. Wester, J. Etula, N. Isoaho, E. Peltola, J. Koehne, M. Meyyappan and others, Mol. Neurobiol., 2020, 57, 179–190.

3 A. Kousar, E. Peltola and T. Laurila, ACS omega, 2021, 6, 26391–26403.

4 Q. Cao, Z. Shao, D. K. Hensley, N. V Lavrik and B. J. Venton, Langmuir, 2021, 37, 2667–2676.

Figure 1

The presence of a carrier selective contact is fundamental for photoelectrochemical (PEC) optoelectronic devices which convert solar energy to chemical/electrical energy. For example, during the overall water splitting (OWS) reaction in an acidic electrolyte, the generated electrons and holes must selectively transport to their respective contacts where they reduce protons and oxidize water, forming H₂(g) and O₂(g) respectively. Often, bare semiconductor (SEM) absorbers are poor catalysts and metals (or metal oxides) (CAT) are alternatively used, although forming a new junction with the underlying SEM. In some cases, the CAT/SEM junction does not theoretically form the required carrier selective junction. A real photocathode consisting of a p-InP and Pt junction should theoretically form an ohmic or non-selective junction, however, this photocathode efficiently reduces protons in operation. In extension, many single OWS particles decorated with cocatalysts theoretically would not form selective contacts producing sufficient open-circuit voltages required to split water (by simple thermionic-emission theory alone). This is further debated in the mechanism for carrier separation during photoelectrodeposition of catalyst in such OWS particles. Here, we exploited conductive probe microscopies and surface-sensitive spectroscopies to reveal the underlying mechanism driving carrier separation towards selective contacts such as alloying transformation and heterojunction formation in the Pt/p-InP system. Additionally, I will demonstrate the unique behavior of nanoscopic electrical contacts by individual characterization (operando and in-situ), demonstrating selectivity by a manifestation of the pinch-off effect. On the Pt/p-InP model system, individual contact selectivity, by the pinch-off effect, on a chemically controlled surface seen in Figure 1.a later evolves due to the formation of an InO_x/p-InP heterojunction after operation (after CV). Furthermore, we investigate SrTiO₃ as a planar model system proxy to OWS particles and show only small variation in the oxygen evolution reaction (OER) current in macroscopic cyclic voltammograms of <100> and <110> terminated surfaces (Figure 1.b). By revealing the underlying mechanisms and the degree that they play, PEC devices can be engineered utilizing an optimal absorber and catalyst whilst their pristine junction would theoretically be non-selective or producing an insufficient photovoltage.

Figure 1

DNA SAMs have received a lot of research interest recently as a suitable platform for modern biosensors. However, there are some shortcomings that remain to be addressed, such as uneven coverage of monolayer, or the spacing between the DNA strands that is hard to control. Here, we present DNA nanostructured cubes as an improved platform for DNA SAM based sensors. The DNA nano-cubes allow for a precise control of ssDNA spacing on the surface and have a great functionalization possibility. We have modified the nanostructures to include thiolated surface anchor and a fluorescence reporter. Cubes were deposited gold electrode via Au-S bonds and analyzed, using AFM and in-situ fluorescence microscopy.

Organically tailorable monolayers of ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAM, Fig. 1a) on Au surfaces are attractive model systems to elucidate the molecular-level physicochemical properties of solid-liquid electrochemical (EC) interfaces [1]. In particular, Fc SAM has been shown to be an attractive spectroscopic molecular probe for photoelectron spectroscopy because the Fe heteroatom is at a fixed position above the electrode surface. Thus, this allows photoelectron spectroscopy to ascertain the electronic and structural properties at a fixed position above the electrode surface [2-3]. The ability to obtain molecular-level structure-function-property relationships with Fc SAM has relevance to bio(chemical) sensors, micro-actuators, and molecular electronics.

Here, we use X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS) to electrochemically control the Fc SAM and spectroscopically probe the corresponding electronic and structural changes [2-3]. To perform our experiments, electrochemical (EC) measurements are performed in a separate and dedicated EC chamber in a solution environment under Ar atmosphere, followed by evacuation and finally transfer to the XPS/UPS analysis chamber [2-3]. The EC chamber contains a retractable PTFE (polytetrafluorethylene) cell and utilizes a hanging meniscus setup. I will discuss the merits and challenges of EC-XPS/UPS methodology in the context of other approaches.

Using EC-XPS/UPS, we are able to discriminate anion dependencies (Fig. 1b-d) with respect to the Fe oxidation state, monolayer thickness, and changes to the valence structure regarding the highest occupied molecular orbital (HOMO. Lastly, we are able to spectroscopically probe the changes relating to the interfacial potential distribution across the electrode-monolayer-electrolyte interface or so-called "potential drop" in the form of systematic potential-induced binding energy shifts in the XPS/UPS spectra. Further details will be provided at the presentation.

Fig. 1 (a) Ferrocene-terminated alkanethiol SAM on Au. Schematic of electrode-monolayer-electrolyte interface and interfacial potential distribution. (b) Cyclic voltammograms at 50 mV s^-1 in 0.1 M NaTFSI or NaPF₆ (c-d) Fe 2p_3/2 spectra of Fc SAM, pristine and after anodic/cathodic polarization at 0.625 and 0 V, respectively.

References

[1] K. Uosaki, Electrochemistry12, 1105-1113 (1999).

[2] R. A. Wong, Y. Yokota, M. Wakisaka, J. Inukai, Y. Kim, J. Am. Chem. Soc.140, 13672-13679 (2018).

[3] R. A. Wong, Y. Yokota, M. Wakisaka, J. Inukai, Y. Kim, Nat. Commun.11, 1-9 (2020).

Figure 1

The underlying principles for generating and storing memory in living organisms differ significantly from traditional hard-matter-based circuits. Bio-inspired membranes are ideal platforms for exploring biomimetic neuromorphic equivalents as they offer novel forms of tunable plasticity and diverse mechanisms to control functionality. Droplet interface bilayers (DIBs) are inherently nanostructures that show electrical properties that are extremely sensitive to nano-scale perturbation. We will demonstrate how dynamic electrochemical impedance spectroscopy (dEIS) can follow molecular-level restructuring in DIBs that lead to hysteretic loops and mem-behaviors in lipid bilayers in response to electrical biasing. We deconvoluted the DIB system's memristance and memcapacitance by measuring the time-dependent complex impedance to show that if the bilayer's structure (thickness or area) changes, these quantities contain the same information; however, if a phase transition occurs, then these responses have additional information. Figure 1, demonstrates both the DIBs platform two major modes of voltage-inducing physical change that can occur in bilayers. Through correlation analysis of the capacitance/resistance, we show that memory processes caused by lipid bilayer expansion, or contraction, do not affect the observed electrical charge/discharge time constant. However, phase transitions resulting in new solvation or lipid structures affect the extracted electrical time constant. In short, dEIS coupled with time-constant analysis provides a means for extracting individual memory elements from a simple two-terminal device, thereby multiplexing the function and potential computational throughput.

Figure 1

Room temperature ionic liquids (RTIL) have had a lot of attention in modern chemical research. Due to their high stability under applied electrode potential, the wide electrochemical window and dual usability as a solvent and an electrolyte RTILs have become very attractive in the field of applied electrochemistry and modern energetics [1]. Molecular self-assembly at solid surfaces, resulting in the formation of the nanostructures with well-controlled properties and functionality reveals fascinating perspectives in science and technology at nanoscale [2]. For instance, the smart tailoring of the structural properties of the functionalized electrodes enables SAMs to be used as protective coatings and for the fabrication of organic thin-film transistors and sensors, and as triggers of specific electrochemical processes.

In this study the in situ STM and impedance spectroscopy methods have been applied to study the structure and properties of the electrochemically polished Sb(111) single crystal electrode | EMImBF4, and EMImBF4 + 1% 4,4´-bipyridine, interface.

Using in situ scanning tunnelling microscopy, the adsorption/desorption of 4,4'-bipyridine was demonstrated and a dense underlying structure, formed below a sparse self-assembled monolayer, was visualized. The detection of two separate adsorbed layers indicates that the ordering of organic molecules could extend well beyond the monolayer on the electrode's surface. These insights are of fundamental and practical importance in the development of nanoelectronics.

References:

[1] H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Inc., New Jersey, 2005, p.1.

[2] T. Wandlowski, Phase Transitions in Two-dimensional Adlayers at Electrode Surfaces: Thermodynamics, Kinetics, and Structural Aspects, A.J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry, Wiley VCH, Weinheim (2002) p. 383-471.

Acknowledgments:

This work was supported by the Estonian Research Council grant PSG249 and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011). For providing us with the computational resources, we would like to acknowledge the HPC Center of the University of Tartu.

The ubiquity of nanomaterials in electrochemical energy conversion technologies has driven intense interest in unravelling the material's structure- function relationships. In response, single- entity electrochemistry has been developed as a technique to study the properties of individual nanoparticles, one at a time. This approach engenders a bottom-up understanding, linking the activities of the single particles to the emergent properties of the ensemble.

The "nano-impact" technique, a subset of single-entity electrochemistry, uses low-noise instrumentation to measure current transients arising from individual particles which stochastically collide with an ultramicroelectrode. These transients have been successfully used to measure the size of both insulating and redox-active nanoparticles, as well as the activity of single electrocatalysts. However, it remains challenging to study certain classes of materials on a single-entity basis. One such class is pseudocapacitors – materials which mix electronic and ionic conductivity, critical components of ion batteries and other next-generation energy storage technologies. While careful experimental design and data analysis have allowed the detection and qualitative characterization of single ion-intercalating particles, their nature as mixed conductors makes quantitative information difficult to obtain amperometrically.

This hurdle is due to the measured current response being dictated by one or more possible factors: electron transfer, ion diffusion within the particle or across the particle/electrolyte interface, or ion transfer in the bulk. To overcome this, electrochemical impedance spectroscopy (EIS) is commonly used to characterize bulk pseudocapacitors. In EIS, a small-magnitude sinusoidal (AC) voltage is applied to the working electrode and the current response is recorded. Different timescales can be probed by varying the AC frequency – the high frequency response is dominated by fast processes, such as capacitance, while the low frequency response contains information on electron transfers and mass transport. In this way, EIS gives a full picture of the electroactive material and can unravel the coexisting electronic and ionic conductivity of mixed conductors.

In this presentation, I will discuss our development of an EIS-based technique to detect and characterize single particles. We implemented fast-Fourier transform based EIS to rapidly acquire impedance spectra spanning several decades of frequencies. The spectra are fit to an equivalent circuit model and monitored as a function of time. Discrete changes in various equivalent circuit parameters are observed, corresponding to single particle-electrode impact events. Using a model system of polystyrene microbeads, we demonstrate that discrete increases in the charge-transfer resistance can be used to accurately measure the contact area between the individual bead and the ultramicroelectrode. I will also discuss our recent progress towards detecting single ion-intercalating nanoparticles and characterizing their charge storage ability on a single particle basis. These advances expand both the scope of single-entity electrochemistry and the depth of information gleaned from a single measurement of a single particle.

In recent years, interest in using unsupported catalysts, especially Pt-based nanomaterials, has resurged in direct methanol fuel cells (DMFC). Unsupported catalysts eliminate the problems related to the corrosion of carbon catalyst supports, thus improving the long-term stability of DMFC. In addition, the design of catalyst synthesis protocols to tailor nanostructured materials with high surface area and catalytic activity improves catalyst performance.¹ Consequently, we recently reported a method for the synthesis of platinum group metal (PGM, i.e., Pt, Pd, Rh) nanoparticles (NPs), using a process called Gas Diffusion Electrocrystallization (GDEx) (Fig. 1a).² The simultaneous electrochemical reduction of CO₂ and water occurs at the triple-phase boundary of uncatalyzed gas-diffusion electrodes, producing H₂ and CO. Both gases, but especially H₂, are reducing agents for water-soluble noble metal ions leading to the formation of small metal nanoclusters. CO can also act as a capping agent. Furthermore, the presence of CO₂ stabilizes the pH, avoiding the formation of metal (hydr)oxides.

In this work, we used the GDEx process to synthesize unsupported Pt NPs using polyvinyl pyrrolidone (PVP, 55000 Mw) as a stabilizer and evaluated their electrocatalytic activity for methanol oxidation. The synthesis was performed at -30 mA cm^-2, using 3.0 mM Pt⁴⁺ (as H₂PtCl₄) as metal precursor and different concentrations of PVP (i.e., 0.0, 0.01, 0.1 and 1.0 g L^-1). We chose low concentrations of stabilizer to facilitate its removal after synthesis, as clean surface catalysts are required for electrocatalytic applications. The size distribution of the Pt NPs, measured using Scanning Electron Microscopy (SEM), was 64 ± 22, 60 ± 22, 42 ± 18, and 38 ± 12 nm for PVP 0.0, PVP 0.01, PVP 0.1, and PVP 1.0, respectively. For comparison, the reduction of 3.0 mM Pt⁴⁺ using 1.0 g L^-1 PVP with only H₂ (either electrogenerated or bubbled) produced bigger and much more polydisperse particles (100 nm–1000 nm), highlighting the importance of the GDEx process and the presence of CO to synthesize small NPs using low concentrations of stabilizer. Furthermore, Transmission Electron Microscopy (TEM) images (Fig. 1b) revealed that the NPs are nanoclusters of single crystals of 2 nm–4 nm in diameter.

The synthesized Pt NPs were cleaned using NaOH,³ and their electrocatalytic activity was evaluated in acidic media. The CV curves of Pt NPs in 0.5 M H₂SO₄ are shown in Fig. 1c. All catalysts showed hydrogen adsorption-desorption peaks from -0.2 to 0.1 V_Ag/AgCl. The calculated electrochemical surface area (ECSA), obtained by integrating the charge in the hydrogen adsorption-desorption region, was 7.6 ± 0.7, 14.1 ± 1.2, 33.6 ± 1.2, and 30.3 ± 1.0 m² g^-1_Pt for PVP 0.0, PVP 0.01, PVP 0.1 and PVP 1.0 respectively. The MeOH electro-oxidation experiments were performed in 0.5 M H₂SO₄ containing 1.0 M MeOH, and the CV curves are shown in Fig. 1d. The forward anodic peak (I_f) at about 0.7 V_Ag/AgCl corresponds to MeOH oxidation, while the backward anodic peak (I_b) at about 0.5 V_Ag/AgCl corresponds to the oxidation of the incompletely oxidized carbonaceous species formed in the forward sweep. The mass activity (MA), defined by the forward peak current density per unit of catalyst loading, was 71 ± 2, 136 ± 7, 463 ± 28, and 341 ± 25 mA mg^-1_Pt for PVP 0.0, PVP 0.01, PVP 0.1, and PVP 1.0 g L^-1, respectively. Hence, the MA scale with the ECSA . Besides, ECSA (and hence MA) increases when the PVP concentration during synthesis increases. However, this trend is not followed for a PVP concentration of 1.0 g L^-1. Even though all catalysts were cleaned using the same protocol, residual PVP might still be left on the surface of Pt NPs synthesized using 1.0 g L^-1 PVP, lowering their electrocatalytic activity.

Overall, the GDEx process was a useful tool for synthesizing unsupported Pt NPs using low concentrations of stabilizer and with high electrocatalytic activity.

This project has received funding from the European Union's Horizon 2020 Research and Innovation program under Grant Agreements n° 730224 (PLATIRUS) and n° 958302 (PEACOC)

1. E. Antolini, J. Perez, J. Mat. Sci, 2011, 46, 4435–4457.

2. X. Dominguez-Benetton, O. Martinez-Mora, J. Fransaer, EP21165681, 2021.

3. A. Zalineeva, S. Baranton, C. Coutanceau, G. Jerkiewicz, Langmuir, 2015, 31, 1605–1609.

Figure 1

Using an electrochemical potential pulse methodology in a mixed solvent system, electrochemical deposition of amorphous vanadium pentoxide (V₂O₅) nanobelts is possible. Crystallisation of the material is achieved using in air annealing with the temperature of crystallisation identified using in-situ heating transmission electron microscopy (TEM). The resulting α-phase V₂O₅ nanobelts have typical thicknesses of 10-20 nm, widths and lengths in the range 5-37 nm (mean 9 nm) and 15 - 221 nm (mean 134 nm), respectively. One-cycle reversibility studies for lithium intercalation (discharge) and de-intercalation (charge) reveal a maximum specific capacity associated with three lithium ions incorporated per unit cell, indicative of ω-Li₃V₂O₅ formation. Aberration corrected scanning TEM confirm the formation of ω-Li₃V₂O₅ across the entirety of a nanobelt during discharge and also the reversible formation of the α-V₂O₅ phase upon full charge. Preliminary second cycle studies reveal reformation of the ω-Li₃V₂O₅, accompanied with a morphological change in the nanobelt dimensions. Achieving α-V₂O₅ to ω-Li₃V₂O₅ to α-V₂O₅ reversibility is extremely challenging given the large structural rearrangements required. This phenomenon has only been seen before in a very limited number of studies, mostly employing nanosized V₂O₅ materials and never before with electrodeposited material.

Figure 1